Summary Currently in clinical practice the detection of acute rejection usually depends on serum laboratory values such as serum creatinine or liver function tests, which tend to be insensitive and non-specific measures of organ injury. Ultimately the diagnosis of rejection requires needle biopsy of the transplanted organ. This invasive procedure is associated with increased cost and potential complications including injury to adjacent organs, bleeding and graft loss. In addition, graft biopsy is limited by sampling error where less than 0.01% of the allograft is examined, as well as inter-observer variability in the interpretation of the result. Patients often require multiple biopsies to assess response to treatment, thus putting them at risk each time they undergo a separate procedure. More recently, there has been significant effort in the development of non-invasive methods to detect rejection including gene expression profiling and -omic analysis of blood and urine samples. Despite recent high impact reports this strategy has not been widely adopted. Here we propose to develop and test non-invasive imaging techniques to monitor the allogeneic immune response to the transplanted organ. Specifically, we will utilize ImmunoPET to track and characterize T cell responses in the setting of CD28 costimulation blockade in vivo. We will use state of the art techniques in mouse models to investigate both nave and memory T cell responses, where we will better define the mechanisms of costimulation blockade resistant rejection. Importantly, our preliminary data in mouse and NHP models reveal a critical role for CD122 in facilitating costimulation blockade-resistant responses; thus, we will use ImmunoPET to visualize the spatiotemporal patterns of CD122+ T cell accumulation following transplantation. We also propose to use ImmunoPET in our non-human primate kidney transplant model to gain further insight into the mechanisms of clinically relevant costimulation blockade-resistant rejection and assess clinical translatability. PET is already widely used in patients and techniques developed in this preclinical setting could be directly applied to clinical practice. This interdisciplinary application combines the expertise of two investigators: Dr. Santangelo who brings exceptional experience using state-of-the-art techniques to image the immune response such as ImmunoPET and Dr. Adams who brings clinical experience and an extensive understanding of novel anti- rejection therapies in transplantation, including costimulation blockade. The use of this powerful imaging technique combined with novel immunotherapies represents a highly innovative approach to further our understanding of the allogeneic immune response following transplantation. In addition, the ability to evaluate this technique not only in rodents but also in a preclinical non-human primate transplant model provides a clear path for potential development and translation of a non-invasive technique to predict rejection in patients.